Epoxy resins have generally excellent dimension stability, mechanical strength, electrical characteristics and further excellent heat resistance, water resistance, chemical resistance, and hence have widely been used for adhesives and coating materials. However, the epoxy resins are usually inferior in flexibility and hence have disadvantageously less peel strength, by which the products prepared therefrom show breaking and peeling during use or processing thereof, and further, in case of a molded product, it is easily broken by giving thereto impact.
From this viewpoint, it has hitherto been tried to improve the flexibility of epoxy resins by various measurements. For instance, it is known that an epoxy resin is reacted with a synthetic rubber which is reactive with an epoxy resin in the presence of a catalyst such as triphenylsulfone, tertiary amines (e.g. dimethylbenzylamine, etc.), tetra-n-butylammonium iodide, and the like (cf. "Zairyo", Vol. 34, No. 384, pages 1099-1104, September 1975, Atsushi Murakami et al, "Phase Separated Structure and Mechanical Properties of Rubber-Modified Epoxy Resins"). Although the modified epoxy resin thus obtained shows satisfactory peel strength and impact strength, the reaction is very unstable and is poorly reproducible because the peel strength of the resin varies depending on each lot of the synthetic rubber and difference of the reaction amount. Moreover, the reaction product has a very high viscosity and hence it is hard to handle.